Low-value resistors, of the order of a few ohm or less, find many applications in semiconductor circuits. One example is that of ballast resistors used as current limiters in bipolar junction transistors (BJTs). BJTs suffer from a phenomenon known as thermal runaway, which derives from the interaction between power dissipation and the temperature dependence of collector current. As a BJT carries current, it converts carrier energy into heat, the quantity of heat generated being proportional to the collector current. Due to the finite ability of the surrounding materials to remove heat, the device temperature increases. Moreover, the physical nature of current transport in BJTs is such that the collector current is exponentially sensitive to the device temperature, a higher temperature corresponding to a higher current for the same base-emitter voltage. This creates a positive feedback, in that an increase in current causes an increase in temperature, which in turn increases the current, and so on. Once thermal runaway is triggered it usually leads to the physical failure of the device. Prevention of thermal runaway requires efficient heat transfer from the BJT, which is sometimes difficult to achieve due to packaging, layout and material constraints.
A simple way to avoid thermal runaway is to add a low-value “ballast resistor” in series with the emitter. The ballast resistor is dimensioned in such a way to damp the positive feedback that causes thermal runaway. For example, a 2-ohm ballast resistor will reduce the base-emitter voltage by 2 millivolt for each additional milliampere of current. Since the emitter current is exponentially sensitive to the base-emitter voltage, such damping effect is generally sufficient to prevent thermal runaway. In addition to their use as ballast resistors, low-value resistors have other applications in radio-frequency (RF) and analog circuits which require resistance values in the range of a few ohm.
Known techniques for the fabrication of low-value resistors have a number of drawbacks. For example, silicided polysilicon normally employed in semiconductor fabrication has a typical sheet resistance of the order of 5-10 ohm/square. The fabrication of a 2-ohm resistor would require a short and wide resistor layout, with a typical width/length ratio between 2.5 and 5. Such wide resistor layout is impractical and uses a large amount of die area. Also, the sheet resistance of silicided polysilicon typically displays large fabrication variations, which result in corresponding large variations in resistance values. Metal interconnects such as aluminum or copper have a typical sheet resistance in the range of 0.05 ohm/square to 0.1 ohm/square. This requires a long and narrow resistor layout, with a typical length/width ratio of 20 or more, often implemented as a serpentine structure which also uses a significant amount of die area.